In a large number of crystals, the center of positive charge does not coincide with the center of negative charge. Such crystals are not symmetric upon reflection and possess, in many cases, physically interesting properties. For example, they may possess a spontaneous electric polarization the magnitude of which depends upon the temperature of the crystal. Such crystals are termed pyroelectric by those skilled in the art.
A simple physical model is usefully discussed to explain the origin of pyroelectricity. Because the centers of the negative charges and positive charges do not coincide, each local structural unit will have a net electric dipole. If the crystal structure is such that the electric dipoles for all units do not cancel each other out, the structure will exhibit a spontaneous electrical polarization. This will happen if the orientations of the dipoles are not random, i.e., the orientations exhibit some degree of correlation. If the temperature of the structure is now increased or decreased, the strength of the dipoles is likely to change because the positions of the positive and negative charges with respect to each other vary as the charges acquire or lose thermal energy. See, for example, Sourcebook of Pyroelectricity, Chapter I, Sidney Lang, Gordon and Breach Co., 1974.
In some pyroelectric materials, the orientation of the electric dipoles in the crystal may be reversed by means of an applied external electric field. Thus, the direction of the electrical polarization may be changed. These materials are termed ferroelectrics by those skilled in the art and form a subset of pyroelectric materials. It will be appreciated that not all pyroelectrics have dipoles which can have their orientation reversed because, for example, the field required for reversal might have a magnitude greater than the electrical breakdown field of the material. Thus, not all pyroelectric materials are also ferroelectric materials.
Several naturally occurring crystals are ferroelectrics. First, there is Rochelle salt together with several chemically related salts. The origin of ferroelectricity in these materials is believed due to the water of hydration in the molecule. Second, there is a set of crystals which includes potassium dihydrogen phosphate and related salts. The ferroelectric behavior in these materials is related to the hydrogen bonds and the motion of the associated protons. Third, there is a class of ionic crystals with either the perovskite or ilemite structure. Barium titanate is an illustrative member of this class. Ferroelectric behavior is observed because the positive and negative charges are displaced with respect to each other and one type of charge has two possible lattice sites.
Ferroelectric and pyroelectric materials are thus fairly common in nature. These materials are of commercial interest because of their dielectric and piezoelectric properties. The materials often have, for example, very high dielectric constants. Other potential applications include temperature sensors and memory devices.